The interactions of drugs used to prevent opportunistic infections with antiretroviral agents is increasingly appreciated. Most studies evaluated two-way interactions despite the fact that most patients receive multiple drugs simultaneously. This study assesses the effect of multiple drugs additively and sequentially on the pharmacokinetics of a hepatically metabolized drug, stavudine. The study design will focus on complex interactions using drugs such as ganciclovir, rifabutin, and fluconazole. The effects of these drugs on each other and on stavudine will be assessed. This study is designed to assess reasons other than resistance that might lead to drug toxicity or efficacy. The study is fully accrued. Data analysis is under way.